Galvanizing process and flux composition



GALVANIZING PROCESS AND FLUX COMPOSITIGN Allan E. Chester, Highland Park,'lll., assignor to'Poor & Company, Chicago, III., a corporation of Delaware No Drawing.", ApplicationMarehj29, r954, SerialNo; 419,597

fofexample, a steel sheet, is passed through an acid pickling bath, rinsed with water, treated with a fluxing agent such as zinc ammonium chloride, dried and then immersed in-molten zinc to form a zinc coating thereon. Themolten zinc bath is usually divided by a partition extending above the surface of the molten zinc and the surface of the molten zinc bath on one side of the partition where the sheet is introduced is preferably providedwith a float flux.

The purpose of the acid pickling step isto remove scale and provide an etch on the surface of the ferrous metal. The flux is applied in'order to clean the surface of the metal and to" provide greater bonding strength between the ferrous metal and the subsequently applied molten zinc. While this general type of process has been used foffinaiiy years, it leaves much to be desired.

one of the objects of the present invention is to provide a new and improved overall galvanizing process which results in zinc coated ferrous metal products characterized by a zinc coating with an enhanced smoothnes's'.

Another object of the invention is to provide zinc coated ferrous metal products characterized by enhanced ductility of the zinc coating.

An additional object of the invention is to provide a new and improved method of producing unusual adherence between zinc and a ferrous metal resulting particular- 1y in improved'bending quality of flexible ferrous metal articles, such as sheets.

Still a further object of the. invention is to provide a galvanizing method characterized by reduced formation of dress.

' Another object of the invention is to provide a new and'improved' process for producing galvanized coatings of zinc on ferrous metals characterized by unusual brillianc'ef Other objects will appear hereinafter.

In accomplishing these objects in accordance with the invention it has been found" that new and improved results in applying molten zinc to ferrous metals are obtained by surface treating the ferrous metal with a coating of a chromium aldonate, preferably chromiumgluconate. The coating of the chromium aldonate is preferably held on the surface of the ferrous metal by a film forming sub stance which adheres to iron, preferably a film of a cationic substance which decomposes in the molten zinc to liberatea free fatty acid.

In the preferred practice of the invention the parts to be'galvanized are immersed in an acid pickling bath containing '3 to 25% by weight of sulfuric acid dissolved -in water, together with a small amount, preferably to 1% by weight of a composition which forms a film t5 atent 011 the ferrous metal containing a chromium aldonate.

f ICC The pickling is preferably effected at a temperature of l80 F. In the pickling operation the ferrous metal sheets or other ferrous metal articles have the scale removed therefrom and the surface etched. At the same time, the film-forming composition containing the chromium aldonate adheres to the surface of the ferrous metal. The parts to be galvanized are then transferred to a rinse tank containing water and also preferably containing a small amount of a chromium aldonate-film-forming composition. The film of material on the surface of the ferrous metal substantially prevents instantaneous formation of oxides during the period of transfer from the acid pickling tank to the rinse tank.

The next step is the transfer of the parts to be galvanized to a flux bath which preferably consists essentially of an aqueous solution of about 1 to about 50% by weight of zinc ammonium chloride dissolved in water. The flux is preferably heated to 100 F.- F. so that when the parts emerge therefrom there is enough heat in them to cause them to dry spontaneously. It is possible, of course, to bake the parts or pass them through heating tunnels in order to effect the drying. In carrying out the present invention a quantity of the chromium aldonatefilm-forming composition is also added to the flux. After the parts to be galvanized pass through the flux they contain a thin film of the chromium aldonate-film-forming composition. They are then passed through a top or floating flux, which provides a foam blanket, into the molten zinc galvanizing bath. While this bath is referred to herein as a molten zinc bath, it will be understood that the term molten zinc includes in this instance the employment of zinc alloys which are sometimes used in this typeof bath as well as the commercial zinc used in galvanizing, commonly called spelter. The chromium aldonate-film-forming composition can also be added to the float flux.

While it is possible to omit the addition of the chromium aldonate-film-forming composition in one or more of the aforementioned steps, the best results are obtained by applying it to the parts to be galvanized, beginning with the acid pickling step and in each step thereafter, thereby insuring that the film containing the chromium aldonate is present at all times during which the ferrous metal is subject to-oxidation;

The chromium aldonate employed in the practice of the invention is the product of the reaction of an aqueous solution of a hexavalent or trivalent chromium compound which forms solutions in the presence of water and alkalis with an aldonic acid. The ratio of the aldonic acid molecule to the chromium atom is preferably at least 1:1 and the product normally exists in the form of a clear green stable aqueous solution. The preferred chromium aldonates employed for the practice of the invention also contain an alkali metal ion and are prepared, for example, by reacting a water soluble chromic compound of an alkali metal with an aldonic acid in proportions such that the ratio of chromium atom to alkali metal atom is within the range-of 1:1 to 1:3, the ratio of chromium atom to the aldonic acid molecule is within the range of 1:1 to 1:3 and the ratio of total alkali metal atom plus aldonic acid molecule to chromium atomis at least 3:1. Examples of aldonic acids which may be used in carrying out this reaction are: gluconic, mannonic, arabonic, galactonic and zylonic. These acids are obtained by the oxidation of the corresponding aldoses, and all of them, including gluconic, have alpha and beta lactone forms which can be present. The chromium compounds which are reacted with the aldonic acid may be, for example, chromic anhydride, sodium'dichromate, sodium chromate, chromic hydroxide, potassium chromate and potassium dichromate. It is preferable tohave a substantial excess ofalkalivpresent during the reaction.

. 3 Chromic aldonates suitable for the practice of the invention are disclosed in U. S. Patent 2,428,356.

The film-forming substance which is employed to provide a carrier for the chromium aldonate can be any one of a large number of'substances which adhere to iron, are capable of forming films and do not interfere with the adherence of the zinc to the iron in the galvanizing bath. Thus, high molecular weight acids derived by the oxidation of petroleum or high molecular weight carboxylic acids derived from vegetable sources can be employed. Oxyalkylated fatty acids are suitable for use in the practice of the invention as, for example, compounds having the general formula in which R is an aliphatic hydrocarbon, hydroxy aliphatic hydrocarbon, or cyclic aliphatic hydrocarbon radical containing at least 8 carbon atoms and sometimes as high as 40 carbon atoms, preferably 12 to 20 carbon atoms, and x is an'integer from to 50. Examples of such substances are the materials derived by the oxyethylation of cocoanut oil fatty acids, oleic acid, hydroxy stearic acid,

ricinoleic acid, stearic acid, and/ or rosin fatty acids (e. g.,

tall oil). These oxyethylated monocarboxylic acid materials are essentially non-ionic.

The preferred film-forming substance employed for the practice of the invention form cationic films and contain nitrogen atoms as a part of the chemical structure of the molecule. Examples of such substances are those having'the general formula r cHzoH.0).n R-N (CHzCHzOhH where R is hydrocarbon radical or a hydroxy hydrocarbon radical containing 8 to 40 carbon atoms linked together, and x and y are integers from 5 to 50. Examples of such substances are those derived by the oxyethylation of fatty amines, including stearyl amine, soya bean amines,

'cocoa amines and tallow amines.

In the non-ionic and cationic oxyalkylated substances previously described, the oxyalkyl groups can also be mixtures of oxyethylene and oxypropylene or all oxypropylene groups, but the number and kind'of such groups should be such that the oxyalkylated substances have wetting properties. The best results have been obtained with materials which are substantially stable in acidic aqueous solutions.

The invention will be further illustrated but is not limited by the following example in which the quantities are stated in parts by weight unless otherwise indicated.

The procedure used was as follows: One-half of the water was heated to 140 to 150 F. The oxyethylated rosin acid material was added with agitation and mixed well with the water for a few minutes until the whitish color had disappeared and the solution had become transparent. p

The oxyethylated soya bean amine was preheated until it was free-flowing and then added slowly-with mixing 4 to the previously prepared solution. The mixing was continued until no stickiness could be felt at the bottom of the kettle with the mixing rod.

The chromium gluconate solution was added and mixed thoroughly with the previously prepared mixture. The balance of the water was then added and the mixing was continued at a temperature of to F.

A suitable chromic gluconate solution can be prepared by dissolving 48 pounds of caustic soda in 10.25 gallons of water and mixing therewith 48 pounds of CrO3. After the mixing is complete, 450 pounds of a 50% aqueous solution of gluconic acid is added slowly. The reaction is highly exothermic and is allowed to proceed for 24 to 48 hours, preferably while holding the temperature below about 200 F. The formation of the chromium gluconate is indicated by a color change from brownish green to a characteristic dark green. The resultant product is adjusted by the addition of water, if necessary, to a product containing 55 grams per liter of Cr.

The composition prepared as above described is suitable for use as a pre-galvanizing flux additive or as an additive to the acid pickling solution in the pretreatment of the metal parts to be galvanized. It preferably has a specific gravity of 1.020 to 1.028 at 20 C., a pH of 8.35 to 8.55 and a chromium content within the range of 1.25 to 1.31, preferably about 1.28 grams per liter.

This composition is added to the acid pickling bath for the parts to be galvanized in a proportion preferably within the range of A to 1% by weight of the acid pickling bath which preferably contains 3 to 25% by weight of sulfuric acid dissolved in water.

The composition prepared as previously described is preferably incorporated with the flux bath by adding .01% by weight to 3% by weight of the composition to the dry zinc ammonium chloride used in preparing the flux bath. This results in a product consisting of Percent by weight Zinc ammonium chloride (ZnNI-I4C1) 97-9999 Additive containing chromium gluconate .01-3

The flux bath is prepared by adding the zinc ammonium chloride containing the chromium gluconate-film-forming additive to water in sufiicient amount to form an aqueous solution containing 1 to 50% by weight ZnNH4Cl. An alternative procedure is to prepare the zinc ammonium chloride bath first and then add the chromium gluconatefilm-forrning additive in proportions corresponding to about .01% to 3% by weight of the ZnNHiCl.

The film-forming substances can be omitted from the composition in the preparation of the flux bath. In the acid pickling bath, however, it is preferable that the filmforming substances and, more particularly, the cationic film-forming substances, be present.

The parts to be galvanized are immersed in the acid pickling bath which is preferably heated to a temperature of 120 to 180 F. until the scale is removed and a finely etched surface is produced. When these parts are removed from the acid pickling bath a thin film containing the chromium aldonate remains on the surface and serves to substantially prevent the instantaneous formation of oxides during the transfer period to the next step which is the rinsing step. The parts are preferably rinsed in water containing a small quantity of the chromium al donate-film-forming additive which is supplemented from time to time so as to insure that the parts leaving the rinse tank are still coated with a film containing the chromium aldonate.

One way of adding the chromium aldonate to the rinse water is to coat a porous block of kaolin with a chromium gluconate-film-forming composition of the type previously described, and insert the resultant porous block in the rinse water where the chromium gluconatefilm-forming composition is gradually dissolved.

The parts are then passed through the flux bath which contains the chromium aldonate-film-formingadditive and is preferably heated to 100 .to 180 F. After leaving the flux bath'the parts'are allowed to dry or are dried by baking them or passing them through heat tunnels and are then ready for immersion in the molten zinc bath. As previously indicate'dgtheyare preferably passed into the molten zinc bath through a float flux which provides afoam-blanketandprevents spattering. The float flux may also contain a small amount, preferably '.0'l% to 3% by weightof the chromium aldonatefilm-forming additive.

In the foregoing example, the composition of the filmforming additive containing the chromium gluconate can be varied considerably without departing from the inven- In the molten zinc bath the film-forming additive normally decomposes to a free fatty acid which acts as a cleansing agent and assists in providing a clean interface between the zinc and the iron. It will be apparent that this is especially advantageous even where the coating applied to the ferrous metal contains no chromium aldonate.

The chromium gluconate acts as a depolarizer in the acid pickling bath and probably also in the zince ammonium chloride flux bath. In any event, the products obtained are characterized by enhanced smoothness, improved ductility of the zinc coating as compared With ordinary galvanized products, thinner zinc coatings, better innermetallic layers at the interface of the zinc and iron, improved bending quality of flexible ferrous metal articles, and unusual brilliance.

The process has the further advantage that the formation of dross is reduced, probably because the film on the surface of the ferrous metal parts to be galvanized prevents the formation of iron salts which, if formed, are carried over into the molten zinc. The presence of such iron salts leads to the formation of a eutective mixture consisting of Zinc and iron which is a waste product and must be removed periodically from the bath.

The invention is applicable to the galvanizing of ferrous metal products which are normally capable of being galvanized, including low carbon steels, high carbon steels and malleable iron forgings.

The invention is hereby claimed as follows:

1. A process of coating a ferrous metal with zinc which comprises precoating the surface of said ferrous metal with a chromium aldonate and applying a coating of molten zinc to said ferrous metal.

2. A process of coating a ferrous metal with zinc which comprises precoating the surface of said ferrous metal with a thin coating of a chromium aldonate dispersed in a film-forming oxyalkylated fatty amine which adheres to the ferrous metal and applying a coating of molten zinc to said ferrous metal.

3. A method of coating a ferrous metal with zinc which comprises precoating said ferrous metal with a thin coating of a chromium aldonate dispersed in a cationic filmforming oxyalkylated fatty amine which decomposes in molten zinc to liberate a free fatty acid and applying a coating of molten zinc to said ferrous metal.

4. A method of coating a ferrous metal with zinc which comprises precoating said ferrous metal with a thin coating of a cationic film-forming oxyalkylated fatty amine which decomposes in molten zinc to liberate a free fatty acid and applying a coating of molten zinc to said ferrous metal.

'6 5. A process of coating a ferrous metal withzinc which comprises "precoating the surface of said ferrous metal with chromium gluconate and applying a coating of molten zinc to said, ferrous. metal; 1

6. A process of coating a ferrous metal-with zinc whichcomprises precoating the surface of said" ferrous metal with a thin coating of chromium gluconate dispersed in a film-forming oxyalkylated fatty amine which adheres to the ferrous metal and applying a coating of molten zinc to said ferrous metal.

7. A process of coating a ferrous metal with zinc which comprises precoating said ferrous metal with a thin coating of chromium gluconate dispersed in a cationic oxyethylated fatty amine which decomposes in molten zinc to liberate a free fatty acid and applying a coating of molten zinc to said ferrous metal.

8. A method of coating a ferrous metal with zinc which comprises precoating said ferrous metal with a thin film of a cationic oxyethylated fatty amine containing about 12 to about 40 carbon atoms and about 5 to 50 oxyethylene groups, and applying a coating of molten zinc to said ferrous metal.

9. A process of coating a ferrous metal with zinc which comprises pickling said ferrous metal in an acid solution, rinsing with water, passing the rinsed ferrous metal through a flux bath, drying the ferrous metal containing the flux, then applying a coating of molten zinc to said ferrous metal and in each of said steps prior to the application of the molten zinc bringing the ferrous metal into contact with a chromium aldonate solution.

10. A process of coating a ferrous metal with zinc which comprises pickling the ferrous metal in a solution of a pickling acid adapted to remove scale and form a fine grained etch on the surface of the ferrous metal, said solution containing a chromium aldonate, thereafter rinsing the ferrous metal with water, passing the ferrous metal through a flux bath, and immersing the ferrous metal in a bath of molten zinc.

11. In a process of coating a ferrous metal with zinc wherein said ferrous metal is treated with an acid pickling bath to remove scale, rinsed with water, passed through a flux bath and immersed in molten zinc, the step which comprises forming a film of a cationic film-forming oxyalkylated fatty amine and a chromium aldonate on the surface of said ferrous metal in said acid pickling bath.

12. In a process of coating a ferrous metal with zinc wherein said ferrous metal is treated with an acid pickling oath to remove scale, rinsed with water, passed through a flux bath and immersed in molten zinc, the step which comprises forming a film of a cationic film-forming oxyalkylated fatty amine and a chromium aldonate on the surface of said ferrous metal in said flux bath.

13. A pregalvanizing additive adapted to form a film on the surface of a metal to be galvanized consisting essentially of a cromium aldonate and a film-forming oxyalkylated fatty amine having wetting properties.

14. A pregalvanizing additive adapted to form a film on the surface of a metal to be galvanized consisting essentially of a chromium gluconate and a film-forming oxyalkylated fatty amine having wetting properties.

15. A flux composition for precoating ferrous metals prior to galvanizing which consists essentially of a major proportion of zinc ammonium chloride and a minor proportion of chromium gluconate.

16. A flux composition for precoating ferrous metals prior to galvanizing which consists essentially of a major proportion of zinc ammonium chloride, a minor proportion of chromium gluconate and a minor proportion of a cationic film-forming oxyalkylated fatty amine.

17. A flux composition for precoating ferrous metals prior to galvanizing which consists essentially of a major proportion of zinc ammonium chloride, a minor proportion of chromium gluconate and a minor proportion of a cationic film-forming oxyethylated fatty amine.

7 18. A flux composition for precoating ferrous metals prior to galvanizing consisting essentially of 97% to 99.99% by weight of zinc ammonium chloride and .01% to 3% by weight of a film-forming composition contain ing chromium gluconate.

8 References Cited in the file of this patent UNITED STATES PATENTS 2,174,762 Mannheim et a1 Oct. 3, 1939 2,428,356 Chester Oct. 7, 1947 2,446,045 -De Groote et a1. July 27, 1948 2,522,937 Forker Sept. 19, 1950 OTHER REFERENCES V Bablik: Galvanizing, E. & F. N. Spon Ltd. (London),

third ed., 1950, p. 147. 

2. A PROCESS OF COATING A FERROUS METAL WITH ZINC WHICH COMPRISES PRECOATING THE SURFACE OF SAID FERROUS METAL IN A FILM-FORMING OXYALKYLATED FATTY AMINE WHICH ADHERES IN A FILM-FORMING OXYALKYLATED FATTY AMINE WHICH ADHERES TO THE FERROUS METAL AND APPLYING A COATING OF MOLTEN ZINC TO SAID FERROUS METAL. 